The installer window is too big to fit on a VGA screen which is used in Safe Mode. Furthermore the normal Windows buttons have been suppressed meaning that the alt-space M trick for moving windows using keys does not work. The grab bar at the top allows a bit of movement with the mouse but that only enough to see the top of the action buttons. Tantalisingly when the mouse is released the window slides down a bit and the buttons move off screen. I auto hid the Windows taskbar out of the way.
That means that the Next button is not visible and cannot be clicked.
My first attempt at an install hung while installing vcredist_x86_sp1.exe. I was following some advice on this forum to use Safe Mode to get round that.
As it happens, that installation completed but avastui.exe would not run. Following some other, older advice I installed MS C++ runtimes before installing avast as a regular user.